In known can lids with a lid ring and a closing membrane arranged thereon, the discharge opening extends up to the vicinity of the outer rim so that after removing the closing membrane, usually formed as a tear film, only a narrow ring surface remains radially adjacent on the inside to the outer rim. After removing (peeling) the membrane, as in a fully tear-off lid, a relatively large container opening results through which the filling is easily accessible. A further advantage of this lid system is that the filling can be sterilized.
In a process known from EP-A-1 153 840 for production of a lid ring for a can lid with closing membrane, first a flat sheet metal part with a heat-sealable coating on one side is formed into a cylindrical tube, the coating being on the inside. The axial edges of the tube are pressed and welded together, which can be achieved by means of a laser beam and leads to the formation of a butt weld seam. The tube is then divided into tube sections or cylindrical rings of equal length. Each cylindrical ring is formed into a lid ring with an outer rim for attachment to a can body and into an annular support surface for subsequent fixing of the closing membrane. Here, the edge bordering the later discharge opening is curled against the side with the heat-sealable coating.
The process described above for production of a lid ring starting from a cylindrical ring has the advantage of a considerable reduction in the amount of waste in comparison with the conventional method of lid ring production in which first a metal disc is molded and then the discharge opening punched out to form the lid ring.
In the process described in EP-A-1 153 840, it must be ensured that, on forming the butt weld seam, the adjacent heat-sealable coating is not damaged. For this, during the welding process a cooling plate is pressed against the inside of the tube along the axial edges. Despite these measures damage to the heat seal layer can never be completely excluded. As the heat seal layer also serves as corrosion protection, corrosion can occur later at damaged points on the weld seam.